$ perl -le'
my $string = q/423RY75Y69827EC67592C78657N965R345U299M/;

print $string;

$string =~ s/(([A-MO-Z])[\dN]+\2)/_$1_/ig;

print $string;

substr( $string, 1 + rindex $string, q/_/ ) =~ s/(?<=[^\dN\3])([\dN]+[A-MO-Z])/_$1/ig;

print $string;
'
423RY75Y69827EC67592C78657N965R345U299M
423R_Y75Y_69827E_C67592C_78657N965R345U299M
423R_Y75Y_69827E_C67592C_78657N965R_345U_299M